This invention relates to flash lamps for underwater photography, inclusive of so-called xe2x80x9cslavesxe2x80x9d which emit light when a so-called xe2x80x9cmasterxe2x80x9d is caused to flash.
Flash lamps are essential in underwater photography both because red light is more easily absorbed by water to cause a color imbalance and because there is usually not enough natural light available. Flash lamps for underwater photography are usually supported at the tip of a flexible arm attached to the camera housing and are frequently oriented obliquely in order to avoid direct reflection of light from plankton and other small animals in the environment. In order to illuminate an approximately rectangular target area while the lamp is obliquely oriented, the lamp is usually required to be able to illuminate an approximately circular area around its optical axis. Prior art flash lamps for underwater photography, therefore, were usually provided with a circularly shaped discharge tube or arrays of prisms arranged over a rectangular panel, in order to illuminate a nearly circular area. A circularly shaped discharge tube for emitting a sufficiently large quantity of light is necessarily large and costly and further requires an equally large reflector, thereby making the lamp unit bulky as a whole. As for lamps with a large number of light-diffusing prisms, means for causing uniformly oriented light to make incidence from behind onto the prism-supporting panel must be provided and this again makes the lamp unit bulky as a whole.
Another unfavorable characteristic of prior art flash lamps for underwater photography has been that a single capacitor with a large capacitance was used for the discharge and a single large transformer was used for operating such a capacitor. Such large electronic components cannot be effectively mounted to a base board to make a compact lamp unit. While miniaturized cameras are coming to be available, flash lamp units for underwater photography remained as large as ever.
Still another unfavorable characteristic of prior art flash lamps for underwater photography relates to the mode switching circuit for switching between the so-called xe2x80x9cautomaticxe2x80x9d and xe2x80x9cmanualxe2x80x9d modes of controlling the exposure. The automatic mode is the mode of operation in which the light emission is terminated automatically when it is ascertained that a specified quantity of light has been emitted, say, by monitoring the reflected light received through the lens of the camera. The manual mode is the mode of operation in which the exposure is manually set by the photographer. In this mode, the photographer may still select whether the light from the lamp is to be used fully or the emission should be stopped at a specified time such that the quantity of emitted light will be a specified fraction, such as xc2xd or xc2xc, of a full exposure. As will be explained in detail below, the prior art switching circuit was bulky, including not only the so-called mode selection switch by which the photographer will select either the automatic or manual mode of exposure but also another switch which was necessary to carry out the switching between the two modes.
This invention relates also to an improved slave lamp of the type which is caused to emit light in synchronism with the light emission from a master lamp. Such a slave lamp comprises a light receiving circuit including at least one light receiving element for receiving light from the master and a signal processing circuit for detecting a start signal element and a stop signal element from the light sensing signal outputted from the light receiving circuit in response to the light received from the master and outputting a start signal and a stop signal respectively for starting and ending emission of flash light from a discharge tube of its own. Explained more in detail, as a start signal element is detected by analyzing the light sensing signal from the light receiving circuit and a start signal is outputted from the signal processing circuit, a trigger circuit for operating the discharge tube is activated. This light sensing signal is also differentiated within the signal processing circuit, and when a stop signal element is detected by differentiating the light sensing signal, a light emission stopping circuit is activated to stop the emission of light from the discharge tube. The light receiving element of such light receiving circuit is usually connected to a low-load resistor in order to prevent saturation because it serves to receive strong flash light with high intensity. As a result, when the light sensing signal from the light receiving circuit is differentiated, the differential signals thus obtained are from several to several tens of millivolts and hence an amplifier becomes necessary. The amplifier and its accessory devices make the flash unit bulky. In addition, the amplifier will amplify noise and other signals not intended to be amplified, thereby increasing the tendency to cause erroneous operations.
There have been problems of other kinds with prior art slave lamps. Since the light receiving element of the light receiving circuit for a slave lamp is generally disposed proximally to the reflective mirror of the discharge tube so as to be able to receive light from the master more effectively, the noise which is generated at the time of triggering the discharge tube is also received through the reflective mirror by the light receiving element and is amplified together with the light sensing signal. Since the high trigger voltage applied to the discharge tube is usually applied electrostatically on the outer surface of the discharge tube in order to activate it, the applied voltage is not completely absorbed by the discharge tube but comes to be discharged in part in air and also into the wiring and the electronic components on printed circuit boards. Such noise may not be harmful to a start signal for starting light emission but, if it contaminates the stop signal, may stop the light emission immediately after the discharge tube begins to emit light.
It is therefore an object of this invention to provide a flash lamp for underwater photography which is compact and light in weight but is still capable of illuminating an approximately circular target area with a relatively large quantity of light.
It is another object of this invention to provide a flash lamp for underwater photography having a compact but reliable mode switching circuit for switching between automatic and manual modes of exposure.
It is still another object of this invention to provide a slave flash lamp having an improved signal processing circuit with a simpler structure for detecting start and end signal elements in a light sensing signals and outputting start and end signals such that emission of flash light from its discharge tube can be started and stopped reliably.
A flash lamp embodying this invention for underwater photography may be characterized as comprising two straight discharge tubes for emitting flash light mounted to a panel perpendicularly to each other, say, in a T-formation or an L-formation such that a circular target area can be illuminated. Since two relatively smaller tubes are used, instead of one large discharge tube bent in a circular form in the attempt to illuminate a circular target area with a large quantity of light, smaller capacitors and smaller transformers which are connected mutually parallel are sufficient and they can be mounted to a panel and a substrate more compactly and in a space-efficient manner inside a watertight housing.
In another aspect of this invention, the mode switching circuit for switching between automatic and manual modes of operation is simplified. With an improved mode switching circuit, only one mode selecting switch is required to be operated on such that an indicator lamp for indicating whether light emission has taken place in the automatic mode or not will function reliably.
A slave type flash lamp embodying this invention may be characterized not only as comprising a light receiving circuit for receiving light from a master flash lamp and outputting a light sensing signal according to the received light intensity and a signal processing circuit for analyzing this light sensing signal to output at appropriate times a start signal for causing flash light to be emitted from its own discharge tube and to stop this emission of light, but also wherein the signal processing circuit includes both a start signal output circuit and a stop signal output circuit for processing and analyzing the light sensing signal to accordingly output a start signal and a stop signal, respectively, for causing light emission from the discharge tube to be started and ended. The stop signal output circuit includes a differentiating circuit for differentiating the light sensing signal to output its differentials, an amplifying circuit for amplifying these differentials and a signal analyzing circuit which analyses the amplified differentials to detect a stop signal element, or a feature in the light sensing signal or its differentials which indicates that a stop signal is to be outputted to stop the emission of flash light. The start signal output circuit serves to analyze the amplified light sensing signal to detect a start signal element, or a feature in the light sensing signal which indicates that a start signal is to be outputted to start the emission of flash light. The light receiving circuit may be divided into two circuits each with its own light receiving element, one of them outputting its light sensing signal to the start signal output circuit and the other outputting its light sensing signal to the stop signal output circuit. In such a case, the two light receiving elements may be used separately or put in a same package. In order to prevent noise effects, the invention also teaches providing a bypass capacitor on the output side of the amplifier circuit in the stop signal output circuit.